1. Field of the Invention
The present invention relates to a surface acoustic wave filter used as, for example, a bandpass filter in a mobile communication apparatus and, more particularly, as an edge reflection type surface acoustic wave filter utilizing an SH-type (shear horizontal wave type) surface acoustic wave, such as a BGS (Bleustein-Gulyaev-Shimizu) wave, or a Love wave.
2. Description of the Related Art
An edge reflection type surface acoustic wave filter of an SH-type does not require reflectors. Therefore, because the filter can be miniaturized, various edge reflection type surface acoustic wave filters have been proposed (for example, U.S. Pat. No. 5,977,686).
In an edge reflection type surface acoustic wave filter, at least one interdigital transducer (hereinafter, referred to as an IDT) is provided. An SH-type surface acoustic wave excited by this IDT is reflected between two opposing edges of a piezoelectric substrate, a standing wave is generated, and a bandpass filter is constructed by utilizing the resonance characteristic based on the standing wave. This type of IDT includes electrode fingers that are interdigitated with each other. In addition to the IDT using so-called single electrodes where electrode fingers connected to different potentials are alternately arranged, an IDT using so-called split electrodes where one single electrode is split into a pair of electrode fingers is also known.
In the above edge reflection type surface acoustic wave filter, insertion loss is dependent upon the type of piezoelectric substrate used. Therefore, where the same piezoelectric substrate is used, it is difficult to substantially reduce loss.
To overcome the above-described problems with the prior art, preferred embodiments of the present invention provide an edge reflection type surface acoustic wave filter in which loss is greatly reduced even where the same piezoelectric substrate is used.
According to a preferred embodiment of the present invention, an edge reflection type surface acoustic wave filter includes a piezoelectric substrate having two opposing edges and at least one interdigital transducer. The interdigital transducer includes split electrodes of paired electrode fingers that are arranged on the piezoelectric substrate such that an SH-type surface acoustic wave is excited on the piezoelectric substrate and is reflected between the two opposing edges. Each of the edges is located at a distance of at least approximately xcex/2xe2x88x925xcex/128 from the center of the paired equipotential electrode fingers, located at the respective outermost portions in the surface acoustic wave propagation direction, of the interdigital transducer and less than about xcex/2 from the center of the paired equipotential electrode fingers, where xcex is a wavelength of the SH-type surface acoustic wave which is excited on the piezoelectric substrate.
It is preferable that each of the edges of the piezoelectric substrate is positioned between about xcex/2xe2x88x92xcex/32 and about xcex/2xe2x88x92xcex/64 from the center of the respective outermost paired equipotential electrode fingers.
In a preferred embodiment, a reflection type surface acoustic wave filter according to the present invention defines a transversely coupled type resonator filter, longitudinally coupled type resonator filter, or ladder-type filter or other suitable filter.
In another preferred embodiment of the present invention, an antenna duplexer including an edge reflection type surface acoustic wave filter according to the present invention is provided.
Furthermore, in yet another preferred embodiment of the present invention, a communication device having the above-described antenna duplexer is provided.
According to preferred embodiments of the present invention, by setting the edges of the piezoelectric substrate at the aforementioned specific location, insertion loss is greatly decreased. Furthermore, by adjustment of the location of the above-described edges, the frequency is adjusted.
When a transversely coupled type resonator filter, longitudinally coupled type resonator filter, or ladder-type filter is constructed according to various preferred embodiments of the present invention, a transversely coupled type resonator filter, a longitudinally coupled type resonator filter, and a ladder-type filter which have low loss are constructed using split electrodes.
In an antenna duplexer constructed by using edge reflection type surface acoustic wave filters according to various preferred embodiments of the present invention, loss in the antenna duplexer is greatly decreased.
Furthermore, in a communication device provided with an antenna duplexer according to a preferred embodiment of the present invention, because an antenna duplexer having low loss is provided as described above, loss in the overall communication device is greatly decreased.
For the purpose of illustrating the invention, several preferred embodiments are shown in the drawings, it being understood, however, that the invention is not limited to the precise arrangements and elements shown.
Other features, elements, advantages and characteristics of the present invention will become more apparent from the detailed description of preferred embodiments thereof with reference to the attached drawings.